This invention relates generally to ball valves and, more particularly, to top entry ball valves employing metal seat rings having a soft metal coating thereon.
Ball valves are well known and generally consist of a valve body having a valve chamber, a substantially spherical valve member or ball positioned in the valve chamber, and one or two seat members positioned between the valve member and the ends of the valve chamber. The valve member has an internal passage therethrough which forms a flow path from valve inlet to valve outlet when the valve is in the open position. Means are provided for rotating the ball from an open to closed position and vice versa.
Several techniques have been employed to accommodate rigid seating members while taking full advantage of top entry construction. For example, the valve chamber may be tapered inwardly from the top resulting in an upper opening sufficiently large for insertion and retraction of the ball and seat assembly. Clearly, precautions must be taken when using this type of arrangement to ensure proper sealing between the ball, seat and valve body. For example, springs must be employed to urge the seats downward into position.
Another known solution employs a ball having an axial length substantially less than its diameter. In this way, when the ball is in the open position, it may freely enter the valve housing from a top opening.
A leakage specification of millimeters of hydrostatic medium has developed for metal seated gate and globe valves which enjoy a large mechanical advantage in the creation of pressures on the valve closure member and seats. In contrast, metal seated ball valves, like check valves, have virtually only the line pressure available to create sealing. Metal seated ball and check valves have been unable to meet the above referred to leakage specifications. In order to achieve even a reasonably tight seal, it has in the past been necessary to lap the balls and seat members to masters and then to each other. This creates, in effect, seat members which are matched to a particular ball and thus the seat members cannot be replaced without replacing the ball. Since the lapping process is both time-consuming and expensive, an alternative approach has been sought.
For example, seat rings for ball valves may be formed, in whole or in part, from various flexible materials, e.g., rubber, Teflon, etc. This is especially suitable for top entry ball valves since the flexible seats are easily deformable, thus permitting easy compression of the seats and valve member and insertion, as a unit, into the valve chamber. However, the use of flexible seat rings has presented certain problems. Most seals are of the compression type, i.e., each seat is compressed between the valve member and an end wall of the valve body causing the spherical surface of the ball to maintain intimate contact with the seat, thereby establishing a seal between the valve body and the seat member and between the valve member or ball and the seat member. Obviously, after a period of time, the flexible materials would sufficiently wear, resulting in a decrease of the initial built-in compression reducing the integrity of the seal. Further, such materials are not suitable for high-temperature service in the order of 1,000.degree. F. or cryogenic service where temperatures in the order of -400.degree. F. must be accommodated. At the higher temperatures, the material simply melts, while at the lower temperature the material cracks. For this reason, metal seat rings must be employed at extreme temperatures; however, their greater rigidity makes top entry a problem. Further, metal seated ball and check valves have been unable to meet the above referred to leakage specifications. In fact, the severity of the above mentioned problems has caused certain manufacturers to request that the leakage allowances of metal seated ball valves be made less stringent.